Environmental protection of a pipe by means of a conductive polymer sheet

ABSTRACT

A conductive polymer wraparound sleeve is used to enclose a joint between district heating pipes. It is wrapped so that longitudinal edges thereof overlap, and it acts as a source of heat to seal the overlapping longitudinal edges. Elongate electrodes, preferably metal bars provided on a carrying strip, are positioned along the overlap and connected to a power source to cause electrical current to flow circumferentially across the overlap region to cause bonding of the overlap.

FIELD OF THE INVENTION

The present invention relates to environmental protection and/ormechanical joining of pipes or cables, particularly thermally-insulatedpipes such as district-heating pipes.

SUMMARY OF THE INVENTION

The present invention in one embodiment provides the use of a conductivepolymer backing sheet in the environmental protection of athermally-insulated pipe to enclose the pipe and to act as a source ofheat to seal overlapping longitudinal edges of the sheet.

In another embodiment the invention provides a method of sealing anelongate object which comprises: wrapping a sheet comprising aconductive polymer backing sheet and a bonding layer around the objectso that longitudinal edges thereof overlap over a longitudinal region;pressing two electrodes into contact with the sheet so that they extendsubstantially parallel to each other along opposite edge portions of thesheet; and powering the electrodes to cause electrical current to flowacross the region to fuse the bonding layer.

DETAILED DESCRIPTION OF THE INVENTION

The product consists of a heat-shrink backing, shrink ratio preferablyfrom 10-50%, for example about 20%. The backing may be electricallyconductive with a resistivity around 1-10 ohm-cm, especially from 2-6ohm-cm. Preferably the material comprises a conductive polymericmaterial. A conductive polymeric material is a composition comprising aconductive filler, typically carbon black, distributed in a polymer,tyrpically an organic polymer. The filler is preferably provided in anamount of 20-60 parts by weight. The polymer may be for examplepolyethylene or EVA. A preferred material is 100-500%, particularly300%, expanded, 0.75-2.5, particularly 1.5, mm thick sheet made inpolyethylene or EVA 30-50%, particularly 39%, carbon filled.

The product is wraparound and can be supplied on the roll or cut tolength in the field or elsewheter. The product can replace oversizedcasings used in district heating joints.

The product can be installed over a cut back between two joined districtheating pipes. A support plate or profile can be installed in the cutback area. This plate can support a longitudinal seal formed duringinstallation of the new sheet.

The sheet can be wrapped around the cut back and closed with a cottonTessa or other suitable tape.

Two metal bars, e.g. aluminium bars, may be used as electrodes. Theseare typically spaced at 25-45, particularly about 35, mm with electricalisolators. The electrodes are positioned over an overlap of ends of thesheet and optionally over a metal or other support. They may be kept inposition by, for example two straps or clamps, etc. These clamps arepreferably positioned over the pipe section, rather than over the cutback area. Another possibility is to provide the electrodes fixed on abacking sheet, so that this provides a separate integral powering devicethat can be strapped to the sheet to power and heat the overlap region,and then optionally removed. Instead of aluminium bars the electrodesmay comprise, for example, electrically conductive braids, or otherstrips.

The product can be installed with a constant wattage or other powersupply, preferably at a power density of 1.5-3, especially about 2.4,Watt/cm².

Application of power causes electrical current to flowcircumferentially, primarily across the overlap region.

No extra pressure need be applied during installation. A surface typethermocouple may be used to regulate the temperature (preferably about155° C.) during installation for a certain dwell time (preferably about60 secs). Circumferential seals can be installed after cool down of thelongitudinal seal.

Any excess amount of material can be cut away next to the electrode ofthe longitudinal seal together with the closing Tessa tape, and, asmentioned above, the electrode optionally removed. A piece of electricalinsulation tape may be glued on the overlap to prevent re-heating of thelongitudinal seal during separate installation of circumferential sealsof the sleeve to the pipe jackets, which is usually carried out. TwoJubilee or other clamps may be used as electrodes. The clamps may bespaced about 35 mm apart. The gaps between the clamps and the sleeves atthe overlap may be filled with metal braid material to reduce contactresistance.

The electrodes may powered with a constant or other wattage supply,preferably about 2-4 Watt/cm² power density.

The electrodes may be re-used after installation, as indicated, ifremoved.

BRIEF DESCRIPTION OF THE DRAWING

The invention is further illustrated, by way of example, with referenceto the accompanying drawings, in which

FIG. 1 shows a joint between district heating pipes;

FIG. 2 shows such a joint enclosed by a new sheet of the invention;

FIG. 3 shows an electrode for use in a new method of the invention; andFIG. 4 shows in cross-section a sheet of the invention;

FIG. 5 shows a support bar installed on a pipe joint prior toinstallation of a sheet.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1 two pipes, for example for conveying hot water for districtheating, etc., are surrounded by thermal or other insulation 2 which inturn is surrounded by a protective jacket 3. Two such pipes are joinedby a weld 4. In order to make such a joint, the insulation 2 and jacket3 of each pipe have to be cut back as shown.

The cut-back jacket has to be made good in order to protect theotherwise-exposed insulation. Also, new insulation may be required to belocated at the join.

FIG. 2 shows a sheet 5 of the invention being installed by a method ofthe invention.

Longitudinal edges of the sheet can be seen to overlap at 6.

Metal or other electrodes 7 and 8 are placed on the overlap, and whenpowered cause the overlap to be sealed. Shrinkage of the sleeve may alsooccur. Circumferential seals to the cut-back pipe jackets 3 may be madein a similar way if desired.

Instead of using two separate electrodes, a single (integral) device maybe placed on the overlap to power the sheet. Such a device is shown inFIG. 3, and comprises a backing sheet 9 and mounted electrodes 7 and 8secured thereto.

The sheet may comprise a conductive polymer material and therefore beself-heating. It may weld together Under the action of heat, but Iprefer that it be coated with a suitable heat-activatable sealingmaterial. The sealing material may be reinforced, for example with afiber reinforcement, e.g. glass fiber. This can provide both strengthand electrical insulation (to prevent any possible short circuiting viathe pipe). Other lining electrical insulating layers may also be used.As shown in FIG. 4, a sheet 5 of the invention can be prepared in whichconductive polymer backing sheet 9 is coated with layer 11 ofheat-activated sealing material reinforced with fibers. Layer 11 servesas a bonding layer.

In order to provide support for the electrodes a support bar 10 may beinstalled bridging the insulated parts of the pipes as shown in FIG. 3.

Since current flows circumferentially between the electrodes,substantially no heat will be produced directly under the electrodes.Even though excessive pressure on the electrodes is not required inorder to achieve good electrical contact, this lack of heat beneath theelectrodes themselves reduces damage to underlying cable jackets. Alsodue to the much greater resistance in the longer current path from oneelectrode to the other around the larger of the two segments of thepipe, virtually all of the heat will be generated where it is needed,i.e. at the overlap region. The sleeve is shown of considerably greaterdiameter than the pipe jacket, and it may be heat-shrinkable and shrinkinto engagement with the jacket on installation. An advantage, however,of the wraparound nature of the preferred sheets of the invention isthat they can if desired be cut to length and/or an overlap can be madeof such a size that the sleeve is substantially tight around the pipesbefore heating. Shrinkage is not then necessary, although a small amountmay still be beneficial.

Any suitable conductive polymer or other material may be used for thesheet. Particularly preferred compositions are those especially suitablefor use in self-bearable heat recoverable articles. Particularly goodexamples are described in European Patent Publication Nos. 307,207 and307,205.

Preferred compositions preferably exhibit a small increase inresistivity on increase in temperature, that is a so-called pseudo PTCeffect. The term "pseudo" is used since although there is an increase inresistivity it is significantly less than that exhibited by materialstraditionally referred to as PTC, and is not to be confused therewith.The composition is selected such that the pseudo PTC effect:

(a) encourages uniform heating of an article, electroded and powered torecovery;

(b) moderates the problem of runaway heating seen with ZTC materials;and

(c) is not sufficiently PTC to invoke hot-lining, even when the articleis laminar and is powered so that current flows substantially parallelto the faces of the article.

Preferably the pseudo PTC ratio, (as hereinafter defined) of therecoverable material is in the range 1.5 to 50 more preferably in therange 11/2 to 10, preferably in the range 2.5 to 5, especially 3. Thepreferred PTC ratios are especially preferred for materials that havebeen expanded 25%-500% (1.25X-6X), especially 25%-300% (1.25X-4X), moreespecially 50%-150% (1.5X-2.5X).

As described before where a material exhibits a PTC effect it exhibits acurl over effect, that is it reaches a peak after which theresistivity/temperature curve changes its slope. After the peak thecurve may decrease in slope, level-off, or fall in resistivity,sometimes with a subsequent rise in resistivity. For the pseudo -PTCmaterials defined herein, the pseudo PTC ratio is defined as the ratioof the "peak" resistivity to the resistivity at 25° C. of the material.The peak resistivity is the highest resistivity where curl-over with adecrease in resistivity occurs (referred to herein as negativecurl-over), or the point of inflection, where a subsequent rise occurs(referred to herein as positive curl-over).

Compositions according to the invention are preferably melt processed.

Resistivity/temperature curves by which the quoted pseudo PTC ratiomeasurements are calculated are obtained using the method set out inEuropean Patent Publication No. 307,205.

Preferred compositions preferably also have a resistivity measured atany particular temperature that increases on recovery. This resistivityincrease occurs at any given temperature in the range 20° C. to T_(e)(where T_(e) is the extrapolated end temperature as measured accordingto ASTM D3418-82). This resistivity increase is additional to anyresistivity change that occurs on recovery as a result of the pseudo PTCeffect. The resistivity change is believed to result from changes in theconductive particle network formed by the filler particles in thecomposition during the recovery process. When the composition is used ina recoverable article this increase in resistivity on recovery, inaddition to the pseudo PTC effect tends to shunt current away from thefirst or more recovered parts during the recovery process.

Preferred articles according to the invention comprise a laminar articleat least part of which has been expanded from X% to Y% to make it heatrecoverable, the article comprising a conductive polymeric material theresistivity of which decreases on expansion, as measured in thedirection of current flow, in at least part of the X-Y% expansion range.X may be zero or finite, and Y is greater than X.

Preferred conductive polymeric compositions according to the inventionare cross linked for example by irradiating with high energy electronsto a beam dose in the range 2-35 MRads, especially 2-25 MRads, forexample 10 to 15 MRads. Cross-linking enhances the recoverable behaviorof the material.

The entire disclosure of the patent publications referred to herein areincorporated herein by reference.

What is claimed is:
 1. A method of environmentally protecting athermally-insulated pipe, said method comprising(a) enclosing the pipewith a conductive polymer backing sheet which comprises electrodes togive an overlap between longitudinal edges of the sheet, (b) positioningthe electrodes over said overlap so that electrical power can be appliedthrough the electrodes between two regions of the sheet that (i) eachextend substantially longitudinally along the sheet and (ii)substantially enclose at least part of the overlap; and (c) connectingelectrical power to said electrodes to generate heat to seal the overlapbetween the longitudinal edges of the sheet.
 2. A method according toclaim 1, wherein the sheet is coated with a heat-activatable sealingmaterial that can be activated by heat generated within the sheetthereby forming the seal.
 3. A method according to claim 2, wherein thesealing material is reinforced.
 4. A method according to claim 3,wherein the sealing material comprises a fiber-reinforced fusion layer.5. A method according to claim 1, wherein the sheet is heat-shrinkable.6. A method according to claim 5 wherein heat generated within the sheetcauses the sheet to shrink.
 7. A method according to claim 1 wherein thepipe is a district-heating pipe.
 8. A method according to claim 1,wherein the electrodes are placed over an outwardly-facing surface of anedge of the sheet.
 9. A method of sealing an elongate object whichcomprises:(1) wrapping a sheet comprising a conductive polymer backingsheet and a bonding layer around the object so that longitudinal edgesof the sheet overlap over a longitudinal region; (2) pressing twoelongate electrodes into contact with the sheet so that they extendsubstantially parallel to each other along longitudinal edge portions ofthe sheet; and (3) applying electrical power to the electrodes to causeelectrical current to flow across the region to fuse the bonding layer.10. A method of sealing an elongate object which comprises,(1) providinga sheet comprising a conductive polymer backing sheet and a bondinglayer around the object so that longitudinal edges of the conductivepolymer backing sheet overlap over a longitudinal region; (2) pressingtwo elongate electrodes which are secured to an elongate backing sheetto form an integral device into contact with the conductive polymerbacking sheet so that the electrodes extend substantially parallel toeach other along longitudinal edge portions of the sheet; and (3)applying electrical power to the electrodes to cause electrical currentto flow across the region to fuse the bonding layer.
 11. A methodaccording to claim 9 wherein the elongate backing sheet is removed afterpower is applied to the electrodes.
 12. A method of environmentallyprotecting a thermally-insulated cable, said method comprising(a)enclosing the cable with a conductive polymer backing sheet whichcomprises electrodes to give an overlap between longitudinal edges ofthe sheet, (b) positioning the electrodes over said overlap so thatelectrical power can be applied through the electrodes between tworegions of the sheet that (i) each extend substantially longitudinallyalong the sheet and (ii) substantially enclose at least pan of theoverlap; and (c) connecting electrical power to said electrodes togenerate heat to seal the overlap between the longitudinal edges of thesheet.
 13. A method according to claim 12, wherein the sheet is coatedwith a heat-activatable sealing material that can be activated by heatgenerated within the sheet thereby forming the seal.
 14. A methodaccording to claim 13, wherein the sealing material is reinforced.
 15. Amethod according to claim 12, wherein the sheet is heat-shrinkable.